Multi-scale understory biomass estimation based on SfM data derived from a low-flying drone under the canopy

Abstract:

Understory vegetation is an important part of evapotranspiration from forest floor. Forest management changes the stand structure and then affects the understory vegetation biomass (UVB). Quantitative measurement and estimation of UVB is a step cannot be ignored in the study of forest ecology and forest evapotranspiration. However, we found a gap in measuring and estimating the UVB at catchment scale. In this study, Structure from Motion (SfM) was adopt simultaneously at two different scales in plantation forest made by Japanese cedar and Japanese cypress with a mean stand height of 16.0 m to reconstruct forest structure from understory to canopy: a) generate canopy height model (CHM) in a 1.1h sub-catchment based on dense pint clouds data derived from a manual low-flying drone under the canopy. b) compute canopy openness data in whole catchment (33.2 ha) based on point clouds of canopy derived from an autonomous flying drone above the canopy. Combined with actual biomass data from field harvesting to build a regression model between the CHM and UVB, which was then used to map spatial distribution map of UVB in sub-catchment. Canopy openness and measured photosynthetically active radiation (PAR) data were used to estimate solar radiation which affects the growth of understory. Then, estimated the catchment scale UVB with the biomass and solar radiation. This approach yielded high resolution understory over catchment scale with a point cloud density of more than 20 points/cm². Strong coefficients of determination (r²=0.75) of the cubic model supported prediction of UVB from vegetation volume. The aforementioned model resulted in an average nRMSE of 23.88%. On spatial distribution map of UVB in the sub-catchment with 1m resolution, the average UVB was 0.82kg/m² and dominated by low ferns. These results reveal the potential of the SfM method in understory structure reconstruction and UVB measurement. This method provides UVB for estimating forest evapotranspiration.